Unfair dismissal?...?

You probably could for the pay during the fourth year. Whether it's worth the effort or not, I seriously doubt it. But would you really want to go back there to work? Do you have the time to stop and deal with a trial? I'd just move on, someone obviously didn't want you there. It doesn't sound like you have grounds to regain employment because that wasn't part of the contract, that is, that at the end of the apprenticeship you would be guaranteed a job.